Methods for deactivating metallic catalyst residues in polymers are known in the art; however, it is desirable to find improved methods for deactivating such residues in order to provide compositions with better color, and less batch-to-batch variation in color.
The most widely-used catalysts for the preparation of stereoregular polyolefins are the Ziegler-Natta bicomponent catalysts. These compositions generally include titanium, vanadium, zirconium, chromium, molybdenum or copper halides in combination with an organometallic compound. [P. C. Hiemenz, “Polymer Chemistry”, Marcel Dekker, NY, pp 488-495 (1984)]. The Ziegler-Natta catalysts exist in many different forms. Most commonly, the catalyst consists of two components, a transition metal compound from groups IVB to VIIIB, in combination with an organometallic compound from groups I to III of the periodic table of elements. Many variations of Ziegler-Natta catalysts have been reported. They may be unsupported, supported on suitable insoluble supports, homogeneous, metallocenes and the like. These are discussed in detail by Manas Chanda in “Advanced Polymer Chemistry”, Marcel Dekker, NY, 2000, pp 742-755 and 791-796. All of these catalyst variations are recognized to exist under the definition of Ziegler-Natta catalysts.
Corrosion of metal process equipment is an additional source of metals in polyolefins. For example, 304 and 316 stainless steels contain iron, manganese, chromium and nickel [“Handbook of Chemistry and Physics, 63rd Edition”, R. C. Weast, Editor, CRC Press, Boca Raton, Fla., pp F-120-F121 (1982)]. The metals from catalyst residues and from corrosion of process equipment can interact with phenolic compounds (antioxidants and ultraviolet stabilizers) used to stabilize the polyolefins during melt-processing, or added to provide improved performance in some end-use application. The phenol-metal complexes are frequently colored, requiring the use of acidic phosphorus compounds as stabilizers to provide acceptable appearance. Acidic phosphorous compounds are typically undesirable in polymers because they can corrode process equipment and form insoluble precipitates with other additives in the polymer [Jan Pospisil and Stanislav Nespurek, “Handbook of Polymer Degradation, 2nd ed., S. Halim Hamid ed., Marcel Dekker, NY, pp 241-242 (2000)].
Polyolefins, including low-density polyethylene, linear-low-density polyethylene, polypropylene and polybutene undergo undesirable oxidation when melt processed in the presence of air. Antioxidants, especially those containing phenol functionality, are widely used to inhibit oxidation during melt-processing, and during end-uses. Many ultraviolet stabilizers also have phenolic functionality. Interaction of these phenolic compounds with metal catalyst residues and with corrosion metals can introduce color into the polyolefins. It would be desirable to develop an additive for improving the color of polyolefins.